1. Field of the Invention
The present invention relates to a method and apparatus for testing a tunnel magnetoresistive effect (TMR) element such as a TMR head element using the tunnel magnetoresistive effect or a magnetoresistive random access memory (MRAM).
2. Description of the Related Art
An evaluation test is in general performed for determining whether or not a thin-film magnetic head with a magnetoresistive effect (MR) head element is a non-defective product, during or after manufacturing the magnetic head. This evaluation test includes a stability test for confirming that any random telegraph noise (RTN) such as Barkhausen noise (BHN) is not produced from the MR head element, and a reliability test for confirming that a breakdown voltage of the MR head element is sufficiently high to withstand an extended period of use.
Actually, for all the MR head elements manufactured, performed is the stability test by using for example a dynamic performance (DP) tester to confirm that no reproduced output of the head elements exceeds a threshold level due to the RTN for a predetermined period of time. However, because the RTN is a burst noise, it may not be generated during the predetermined period of time. Even if the RTN is generated, it may not be detected due to its level lower than the threshold.
Japanese patent publication No. 2000-260012 discloses a testing method of a stability of a normal MR head element but not a TMR head element. In this method, change in the reproduced output from the MR head element is evaluated by repeatedly executing a step of applying both AC record current and external DC magnetic field to the MR head element and a step of measuring the reproduced characteristics after that application.
However, this testing method not only additionally needs a magnetic field generation device for applying the external DC magnetic field to the MR head element but also adversely affects a bias magnetic field of the MR head element due to the application of the external magnetic field. On top of that, it is difficult to detect the burst noise RTN with reliability even by this testing method.
As will be noted, the conventional reliability test of measuring a breakdown voltage of the magnetic head cannot be performed for all the magnetic heads because it breaks the magnetic head during the breakdown voltage measurement.
In order to solve such problems, the applicants of this application have proposed a testing method and apparatus for evaluating whether a TMR element is non-defective product or not by feeding a current through the TMR element from the substrate side or lower side to the anti-substrate side or upper side, and by comparing a rate of change from its initial resistance to its resistance after continuously feeding the current with a threshold (Japanese patent application No. 2004-062031, filed on Mar. 5, 2004, U.S. patent Ser. No. 11/070,237 by Shunji et al., filed on Mar. 3, 2005).
However, according to this testing method and apparatus, because it is necessary to determine the threshold value of the resistance-changing ratio each time the period for feeding the current, the voltage applied to the TMR element and the material for the TMR element change, the testing procedure becomes very complicated. Therefore, it is very difficult to evaluate whether each TMR element is a non-defective product or not.